There is an increasing interest in the use of ultrasonic velocity and attenuation (Broadband Ultrasonic Attenuation, BUA) measurements in human bone, particularly the human calcaneum (heel) to identify and monitor osteoporotic changes.
A known ultrasonic assessment apparatus is illustrated diagrammatically in FIG. 1 of the accompanying drawings. The apparatus comprises a primary ultrasonic transmitting transducer 1, a primary ultrasonic receiving transducer 2 and a vessel 3 containing a suitable liquid 8, typically water, through which the transmitted ultrasonic signal 9 can propagate.
An electrical signal generator 4 is connected to supply a short voltage pulse e.g. 600 V, 1 .mu.s , to the input of the transmitting transducer 1. The output 10 from the receiving transducer is fed to an analyser 11 comprising a digitising unit 5 and a processor 6 programmed to perform a Fast Fourier Transform (FFT) on the digitised data. The processor 6 is also programmed to analyse the resultant frequency spectrum to produce a plot of attenuation as a function of frequency which may then be printed out using a plotter 7 or the like.
The slope of the attenuation/frequency plot expressed in dBMHz.sup.-1 is known as the BUA value and provides an indication of bone condition.
In use, the patient's heel 12 is placed in the vessel 3, as shown, the equipment operates and the frequency spectrum is measured. In this way, the attenuation of the transmitted ultrasonic pulse due to absorption by the patient's heel 12 can be measured over a desired frequency range. Typically, measurements are made over the frequency range from 0.2 MHz to 0.6 MHz. For the human heel, the slope of a plot of attenuation against frequency may lie in a range from about 20 dBMHz.sup.-1 (a BUA value of 20) to 120 dBMHz.sup.-1 (a BUA value of 120), depending on bone condition, with a typical attenuation of about 10 dB at 0.2 MHz.
In an alternative, "dry", design of ultrasonic assessment apparatus as shown in the applicants prior U.S. Pat. No. 5,452,722 (the contents of which are incorporated herein by reference), the ultrasonic transmitting and receiving transducers 1, 2 are positioned directly against the patient's heel using soft coupling pads to accommodate variations of heel shape.
In order to compensate for any frequency dependency of the transducer response the frequency spectra obtained are initially corrected by subtracting a reference spectrum derived from a reference material, such as de-gassed water. In the case of an assessment apparatus comprising a water-filled vessel, the reference spectrum may be updated regularly.
In order to monitor the reliability and repeatability of the measurements it is necessary to provide a standard (a so-called quality assurance phantom) which simulates the attenuation properties of bone.
However, such quality assurance phantoms tend to be unreliable, especially when they are used to simulate the attenuating effect of cancellous bone. Furthermore it is very difficult to accurately simulate bone properties using a physical substitute.
Epoxy-resin models of CAD heel structures have been produced using stereo-lithography techniques. Models produced in this way have a highly repeatable structure; nevertheless, they are found to exhibit an inter and intra phantom precision (i.e. a coefficient of variation) of about 4%, a value which is considered to be too high.
In an alternative design, bone structure is simulated in a model by gelatine pellets randomly dispersed in epoxy resin. However, this design has the drawback that there is very little control over the internal structure of the model.